This invention relates to an imaging process, and an imaging medium for use in this process. More specifically, this invention relates to an imaging process in which the image is formed by the change in cohesivity of a polymer upon depolymerization thereof.
Various imaging systems which rely upon the changes in physical properties accompanying polymerization of a monomer or oligomer are known. For example, U.S. Pat. No. 3,890,152 and U.S. Pat. No. 3,487,764, among others, describe a process in which a photosensitive layer is imagewise exposed, thus becoming less tacky, and then a powdered toner is spread over the photosensitive layer so that it adheres only to the unexposed, tacky regions of the layer, excess toner being mechanically removed from the layer. This type of process presents difficulties in handling the finely powdered toner required. In addition, the tacky surface of the unexposed photosensitive layer is highly susceptible to contamination with dust, fibers, fingerprints and the like, and any such contamination may cause unwanted artifacts in the image produced. Finally, in practice it is difficult to remove all the toner from the supposedly non-tacky areas of the image, since the toner may tend to adhere to these areas because of residual tackiness or electrostatic forces.
U.S. Pat. No. 4,356,252 describes a negative working tonable element having a photosensitive layer comprising a binder, a photosensitive acid generator and a polymeric or monomeric acetal. Upon exposure, the photosensitive acid generator generates acid, thereby causing depolymerization or polymerization of the acetal, so rendering the exposed regions tacky. The exposed photosensitive layer is then dusted with powdered toner, which adheres only to the tacky exposed regions, to form an image. This process suffers from problems with toner handling and toner removal similar to those experienced by the processes discussed in the preceding paragraph.
U.S. Pat. No. 4,910,120 describes a color proofing process in which a colored photosensitive layer containing a polymerizable material and a polymerization initiator is disposed between two other layers such that upon imagewise exposure of the photosensitive layer and peeling apart of the two other layers, the exposed regions stay with a first one of the two other layers while the unexposed regions stay with the second one of the two layers.
U.S. Pat. Nos. 4,145,216 and 4,081,282 both describe a dry transfer image system. This system comprises a first carrier film, a tacky photopolymerizable layer, a colored layer and a second carrier film. Upon imagewise exposure through the first carrier film, the photopolymerizable layer photopolymerizes in the exposed regions, thereby rendering it less tacky. Thereafter, the two films are separated, so that in the unexposed regions the colored layer remains with the first film, but in the exposed regions the colored layer remains with the second film.
U.S. Pat. No. 4,806,451 describes a color proofing process in which a photosensitive layer is exposed to produce imagewise tacky and non-tacky areas, and the exposed layer is brought into contact with a non-photosensitive transfer layer containing a finely divided powder or dissolved dye, so that the transfer layer adheres selectively to the tacky areas.
Imaging processes which rely upon polymerization of a monomer or oligomer suffer from several problems. In practice, it is difficult to secure complete polymerization within the limited exposure periods required in imaging. As a monomer or oligomer polymerizes, its viscosity increases, and this rise in viscosity reduces diffusion of monomer or oligomer to polymerization centers. Consequently, as the degree of polymerization increases, the polymerization reaction slows down, and thus it is difficult to remove the last traces of monomer or oligomer from the polymerizing mixture.
Polymerization reactions are often influenced by environmental conditions. Free radical polymerization is inhibited by oxygen, while acid catalyzed cationic polymerization is inhibited by water and other nucleophiles, such as halide ions. In practice, it is usually necessary to store an imaging medium for substantial periods between its production by the manufacturer and its use by a customer, and unless elaborate and expensive precautions are taken, atmospheric oxygen and water may be present in unpredictable amounts in the polymerizable layer of the imaging medium when it is used. Furthermore, monomers useful for the production of polymers tend to be highly reactive, and hence tend to be toxic, volatile and unstable under prolonged storage. Consequently, it is difficult to control the sensitivity and reproducibility of an imaging medium which relies upon polymerization for formation of an image.
Imaging systems are known which rely upon depolymerization of a polymer rather than polymerization of a monomer or oligomer, and several such depolymerization systems are described in M. W. Ranney, Specialized Curing Methods for Coatings and Plastics, Noyes Data Corporation, Park Ridge, N.J. (1977). For example, in one such system described by Ranney and in U.S. Pat. Nos. 3,915,704 and 3,917,483, an acid-degradable polymer is admixed with a photochemical acid generator, so that, upon exposure of the mixture to radiation, generation of acid and consequent degradation of the polymer will produce a visibly discernable image.
Both patents mention that, if only about 25-40% of the acid degradable polymer is present, degradation of the polymer results in plasticizing the exposed regions, which can then be selectively removed by contacting the exposed polymer layer with a sheet of paper, which pulls the exposed regions away from the unexposed, the paper thereafter being treated with a conversion fluid to form a printing plate. Finally, both patents discuss the use of color-forming reagents in conjunction with the acid generator and polymer to form colored images.
International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237; the disclosure of this Application is incorporated herein by reference) describes a thermal imaging medium and a process for forming an image in which a layer of a porous or particulate imaging material (preferably, a layer of carbon black) is deposited on a heat-activatable image-forming surface of a first sheet-like or web material (hereinafter the "first sheet element"), the layer having a cohesive strength greater than its adhesive strength to the first sheet-like element. Portions of this thermal imaging medium are then exposed to brief and intense radiation (for example, by laser scanning), to firmly attach exposed portions of the imaging material to the first sheet element. Finally, those portions of the imaging material not exposed to the radiation (and thus not firmly attached to the first sheet element) are removed, thereby forming a binary image comprising a plurality of first areas where the imaging material is adhered to the first sheet-like element and a plurality of second areas where the first sheet-like element is free from the imaging material.
In a preferred embodiment of the imaging medium described in the aforementioned International Application, the imaging material is covered with a second laminated sheet-like element so that the imaging material is confined between the first element and this second element. After imaging and separation of the second element (with the unexposed portions of the imaging material) from the first element, a pair of images is obtained.
A first image comprises exposed portions of image-forming substance more firmly attached to the first element by heat activation of the heat-activatable image-forming surface. A second image comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element.
In the copending application Ser. No. 07/616,982, filed Nov. 21, 1990 by Kuang C. Chang entitled "Thermal imaging medium", there is described a variation of the process described in the aforementioned International Application; in this variation, a thermoplastic intermediate layer is disposed between the heat-activatable image-forming surface and the layer of porous or particulate imaging material. On separation of the sheets after thermal imaging, the thermoplastic intermediate layer adheres preferentially to the surface of the image formed by the non-exposed porous or particulate imaging material (i.e., the intermediate thermoplastic layer fractures and transfers with the porous or particulate imaging material) and provides surface protection for this image and improved handling, durability and abrasion-resistance characteristics.
In the aforementioned preferred embodiment of the process described in the aforementioned International Application (in which the imaging material is sandwiched between two sheet-like elements), it is usually highly desirable to provide a release layer between the imaging material and the second element in order to facilitate release of the exposed areas of imaging material from the second element, since such facile release of the exposed areas helps to ensure a high quality image. Furthermore, for technical reasons, in some applications it is desirable to use a "write white" imaging process, that is to say a process in which the exposed areas appear white in the final image. In the process described in the aforementioned International Application, it is the image formed on the second element which is the "write white" image; hence, where an image of such character is desired, the image on the second element can be retained and used. The second element still contains the release layer beneath the areas of imaging material, and this release layer may be the weakest layer of the image. Consequently, if the image on the second element is subjected to severe mechanical stress, in some cases the image may tend to delaminate at the release layer.
This invention relates to an alternative to the process described in the aforementioned International Application in which, instead of exposure firmly attaching an imaging material to a heat activatable surface, image formation is caused by loss of cohesive strength in a depolymerizable layer. The present invention permits the formation of a "write white" image which does not contain a release layer, any release layer present being discarded in the "write black" image.